The present invention generally relates to silicon germanium (SiGe) technology, and more particularly to a silicon germanium carbon heterojunction bipolar transistor (SiGeC HBT) for use in various electronic devices.
Description of the Related Art
Silicon Germanium (SiGe) technology has become mainstream in today""s RF (radio frequency) applications, high speed wired data transmission, test equipment, and wireless applications. However, two limitations exist in conventional SiGe HBT devices.
First, the silicon germanium alloy film must remain below a critical thickness. The relationship for the critical thickness follows different physical models, such as the People and Bean, and Stiffler models. These theoretical models demonstrate the relationship of the allowed critical thickness of the silicon germanium film as a function of the germanium concentration. These models indicate that, as the germanium increases (from 0% to 100%), the critical thickness of the film decreases. The critical thickness of a film is the thickness where misfit dislocations are initiated. The transition of a materially stable film to an unstable film is the point where the misfit dislocations are formed.
Because germanium falls below silicon in column four of the Periodic Table, it has a larger lattice constant than silicon. Thus, free-standing silicon germanium will have a larger lattice constant compared to the silicon lattice. One may fit this larger lattice constant silicon germanium material on the smaller silicon substrate by accommodating the difference in the lattice constant through the introduction of misfit dislocations. This silicon germanium film is called the relaxed layer. Additionally, one may grow a layer of silicon germanium on silicon by compressing the horizontal lattice constant of the silicon germanium film to fit on the substrate silicon lattice sites without the introduction of misfit dislocations. This compression of the horizontal silicon germanium lattice constant leads to an increase in the vertical lattice constant. This silicon germanium film is called a xe2x80x9cstrainedxe2x80x9d or pseudomorphic silicon germanium film. Increasing the germanium concentration increases the strain in the layer, hence limiting the allowed thickness of the film. Therefore, a solution is required to allow for a means to relieve the strain in order to increase either the germanium concentration or the film thickness.
Thus, there remains a need for a SiGe HBT device which overcomes the limitations of the conventional devices, such as the critical thickness requirement of silicon germanium film, and the outdiffusion of the base dopants which limits base width scaling.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional silicon germanium heterojunction bipolar transistor devices, the present invention has been devised, and it is an object of the present invention to provide a structure for a silicon germanium heterojunction bipolar transistor device having substitutional carbon dopants in the silicon germanium alloy film. It is another object of the present invention to provide a device which allows for electrostatic discharge protection. Still another object of the present invention is to provide a device which utilizes a tighter statistical distribution of the electrostatic discharge failure voltage. Yet another object of the present invention is to provide a device which uses carbon to provide a larger margin of thickness to allow for a higher thermal strain prior to dislocation formation. It is still another object of the present invention to provide a heterojunction bipolar transistor device which relieves the strain in a silicon germanium alloy film of the heterojunction bipolar transistor device which increases either the germanium concentration or the film thickness. Another object of the present invention is to provide a heterojunction bipolar transistor device, which provides a tighter distribution of sheet resistance, base widths, and breakdown voltages. Still another object of the present invention is to provide a heterojunction bipolar transistor device which controls the outdiffusion of boron. Yet another object of the present invention is to provide a device which achieves a higher unity current gain cutoff frequency (fT) and unity power gain cutoff frequency (fMAX).
In order to attain the objects suggested above, there is provided, according to one aspect of the invention, a SiGe HBT device with carbon incorporation. Specifically, by adding small amounts of substitutional carbon dopant to a Si1-xGex layer, the critical thickness requirement is relaxed for the People and Bean, and Stiffler models, previously described. Because a carbon atom is smaller than a silicon atom, the stress introduced by the germanium atom can be relieved by a smaller atom. Hence, carbon compensates for the strain introduced in the film by germanium. Thus, using a device which introduces a lower initial strain condition allows for a higher thermal strain prior to the initiation of misfit dislocations. Adding carbon to the base region of a silicon germanium epitaxial film allows for a more thermal robustness during an electrostatic discharge event. This is true for bipolar transistors and associated elements that can be constructed, such as varactor structures, pin diodes, and other passive elements formed in this film.
Moreover, carbon provides suppression of the transient enhanced diffusion of boron base dopant outdiffusion in SiGe HBT devices. The importance of this effect allows extensions of SiGe HBT devices to achieve a higher unity current gain cutoff frequency (fT) and unity power gain cutoff frequency (fMAX) Electrostatic discharge sensitivities and electrostatic discharge implications of the silicon germanium carbon (SiGeC) heterojunction bipolar transistor, or carbon incorporation into the base of a silicon germanium film, will lead to a tighter base width control, and, hence a tighter breakdown distribution.
During an electrostatic discharge (ESD) event, significant increases occur inside the SiGe HBT device""s silicon germanium film. As the temperature of the film increases, an additional thermal strain can be initiated. The increase in thermal strain is an additive to the pre-existing strain in the film. Moreover, thermal strain is proportional to temperature. Hence, it would be an advantage to provide a means to reduce the total strain in the film during an electrostatic discharge event by reducing the initial strain in the pseudomorphic silicon germanium film so that misfit dislocations are not generated during an electrostatic discharge event.
Also, base dopants outdiffuse, which limits the base width scaling. One advantage of a heterojunction bipolar transistor is the ability to have a much higher base doping concentration compared to homojunction transistors. One of the major constraints related to conventional SiGe HBT devices is the boron outdiffusion in the base region. With the high doping concentration, the base doping concentration can exceed the emitter doping concentration by an order of magnitude, and can exceed the collector doping concentration by 2 to 3 orders of magnitude. As a result, when boron outdiffusion occurs, the base dopants compensate the emitter and collector regions leading to larger base widths. As the base width increases, the transit time across the base increases, leading to slower transistors. Hence, there is a limit to how much dopant can be achieved within the base region because the low sheet resistance can be compromised by the larger base widths. Boron diffusion is increased in silicon because of transient enhanced diffusion (TED) effects due to the excess of interstitials created by implantations. Boron transient enhanced diffusion plays a role in the outdiffusion of the Boron dopants during hot processing. Because the population of excess interstitials can vary statistically, this can lead to a wider distribution and poorer control of the outdiffusion. Combining this effect with hot process variations (e.g., temperature control during hot processing), the base resistance and the base width can vary in a SiGe HBT device with a heavily doped boron base region.
The variations in base width leads to statistical variations in the unity current gain (fT) and unity power gain (fMAX) The variations in the distribution can lead to worst case radio frequency parameters as well as a lower breakdown voltage. When an electrostatic discharge event occurs, if the distribution of the breakdown voltages translates to a lower second breakdown or thermal runaway, this leads to a degradation in the electrostatic discharge robustness of the transistor element. Hence, it would be valuable to be able to provide a means of providing a tighter distribution of sheet resistance, base widths, and breakdown voltages by controlling the outdiffusion of boron.
Therefore, a novel silicon germanium heterojunction bipolar transistor device is disclosed that comprises a semiconductor region, and a diffusion region in the semiconductor region, wherein the diffusion region is boron-doped. The semiconductor region comprises a dopant therein to minimize boron diffusion. A combination of the amount of the dopant, the amount of the boron, and the size of the semiconductor region is such that the diffusion region has a sheet resistance of less than approximately 4 Kohms/cm2. The dopant comprises carbon. Also, the diffusion region is boron-doped at a concentration of 1xc3x971020/cm3-1xc3x971021/cm3. Preferably, the semiconductor region comprises 5-25% germanium and 0-3% carbon. The device further comprises a collector structure connected to a base region, wherein the base region comprises the diffusion region.
Alternatively, a device is disclosed that comprises a semiconductor substrate and a plurality of bipolar transistors on the semiconductor substrate. Each of the bipolar transistors comprises a base region having a base resistance. One of the bipolar transistors has a base resistance below approximately 4 Kohms/cm2.
Still alternatively, a transistor structure is disclosed that comprises a substrate, a collector region in the substrate, an epitaxial base region on the collector structure containing a Si1-x-yGexCy compound, an emitter region on the epitaxial base region, and a boron-doped base implant diffusion region, wherein the base dopant implant diffusion is suppressed by the Cy concentration. In the above chemical formulas, subscripts x and y are indicated as percentages.
Moreover, by adding carbon to the semiconductor region, the device achieves an electrostatic discharge robustness, which further causes a tighter distribution of a power-to-failure of the device, and increases a critical thickness and reduces the thermal strain of the semiconductor region.